Ground fault interrupter

Abstract

A ground fault interrupter for two and three wire, single phase load circuits employing a circuit breaker having an electro-responsive contact trip-open mechanism, a fault sensing differential transformer, an operational amplifier providing characteristic output pulses whose magnitude depends upon the non-presence or presence of a ground fault and whether the latter is resistive or reactive in kind, and an energizing circuit for the electro-responsive contact trip-open mechanism including a transistor which is normally held non-conducting in the absence of a ground fault and rendered conducting whenever a resistive ground fault occurs. A capacitor in circuit with the amplifier output and transistor base renders the transistor immune from amplifier pulses characteristic of reactive ground faults. Varistors are included between the load circuit wires to suppress transient voltage surges from falsely inducing nuisance ground fault interrupting action.

We claim: 
     
       1. Ground fault interrupter apparatus, exhibiting reduced sensitivity to capacitive ground faults and suitable for economical and compact fabrication, comprising: a differential current transformer having a magnetic core for association with primary windings that are conductors of an electrical system to be protected by the apparatus and also having a sensing element for sensing current unbalance between the primary windings;   an amplifier having at least one input terminal connected with said sensing element, and   bias supply means for supplying to said amplifier a bias potential that is periodically zero and comprising a rectifier for half wave rectifying AC line voltage and the half-wave rectified potential produced by said rectifier is supplied to said amplifier free of filtering.   
     
     
       2. Ground fault interrupter apparatus in accordance with claim 1 wherein: said amplifier has an output terminal, and;   a solid state switch is provided having a control terminal connected with said output terminal of said amplifier.   
     
     
       3. The subject matter of claim 1 wherein: said means to supply an operating supply to said amplifier comprises an A.C. source and a diode rectifier; said thyristor is connected by said main terminals in series aiding direction with said diode rectifier. 
     
     
       4. Ground fault interrupter apparatus in accordance with claim 2 wherein: said solid state switch is in a circuit branch connected between conductors of an electrical system to be protected by the apparatus in series with said rectifier and with a solenoid trip coil of a circuit breaker to be tripped upon occurrence of a ground fault.   
     
     
       5. Ground fault interrupter apparatus in accordance with claim 4 wherein: said solid state switch has a rectifying characteristic and is in said circuit branch in series aiding direction with said rectifier.   
     
     
       6. In a ground fault interrupter for insertion between an alternating current supply circuit and an electrical load, said circuit having a neutral, electrically grounded conductor and at least one electrified conductor, the combination with a circuit breaker having contacts connected in circuit with each electrified conductor and having an electro-responsive trip open mechanism, and a differential transformer having primary windings connected in series with each of said conductors and having a secondary winding, (a) an operational amplifier having its two input terminals connected in circuit with said transformer secondary winding,   (b) an integrating capacitor in circuit with the output terminal of said amplifier   (c) a transistor having its base connected in circuit with the output of said amplifier and said integrating capacitor and being normally biased to cut-off when no ground fault signals are induced, or when reactive ground faults signals are induced in said transformer secondary winding, said transistor being rendered conducting whenever a resistive ground fault signal of a predetermined level is induced in said secondary winding   (d) means in circuit with said alternating current supply and supplying half wave rectified D.C. bias to said operational amplifier and the base and emitter of said transistor, and   (e) means responsive to conduction of said transistor to energize said electro-responsive contact trip open mechanism.   
     
     
       7. The combination according to claim 6 wherein said means responsive to conduction of said transistor comprises a semi-conductor controlled rectifier having its main conducting path connected in circuit with said electro-responsive contact trip open mechanism and its control electrode in circuit with the output side of said transistor. 
     
     
       8. The combination according to claim 6 wherein said means responsive to conduction of said transistor further comprises circuit means latching the base of said transistor to a reference voltage maintaining it conducting in response to conduction of said semi-conductor controlled rectifier. 
     
     
       9. A ground fault interrupter as defined in claim 6 together with a capacitor connected across said transformer secondary winding to tune out any inductive effects being superimposed on the signal input terminals of said amplifier, a second capacitor connected between one side of said transformer secondary winding and one input terminal of said amplifier, and a field effect transistor having its drain electrode connected to the point common between the last mentioned capacitor and the signal input terminal of said amplifier, its source electrode connected to the opposite side of said secondary winding and its gate electrode connected to said neutral conductor. 
     
     
       10. A ground fault interrupter as defined in claim 6 wherein there are two electrified conductors, wherein the differential transformer is provided with three primary windings connected in series with each of said electrified conductors and said neutral conductor, respectively, wherein said amplifier provides output pulses of opposite polarity each half cycle of the A.C. supply, and wherein a second transistor is provided which is biased by such output pulses on the opposite half cycle pulses from those biasing the first mentioned transistor and wherein the means responsive to conduction of said first mentioned transistor is also responsive to conduction of said second transistor to energize said electro-responsive contact trip-open means of said circuit breaker.